Buying well · 13 of 19
What does an A++ window rating actually mean?
A window energy rating is a single letter summarising a calculation that combines heat lost through the window, heat gained from sunlight through it, and air leakage around it. Building regulations are not satisfied by that letter. They are satisfied by the U-value, which measures heat loss alone, so a rating and a compliance figure are two different numbers and a quote should give you both.
Ratings are useful for comparing two windows quickly. They are not what your installer is certifying against, and the difference between the two is where most of the confusion in window sales lives.
What goes into the letter
The rating is produced by a scheme, the best known being the BFRC, and it balances three things:
- Heat lost through the window, which is the U-value.
- Heat gained from sunlight passing through the glass.
- Air leakage through the window as a manufactured unit.
Solar gain is the reason a rating can move in ways that feel counter-intuitive. Adding a third pane lowers the U-value but also cuts the sunlight coming through, so a triple-glazed window can carry a similar or even lower rating than a good double-glazed one while genuinely losing less heat. Neither number is lying; they are measuring different things.
The number that actually has to be met
For a replacement window in England and Wales, compliance is a U-value, and it is the figure your installer certifies against when they register the work. The rating letter has no role in that.
So when you compare quotes, the question is not "what rating is it". It is: what U-value is the whole window, and is that the window or just the glass? A glass-only figure is always better than the whole window including its frame, and quoting the glass figure as though it were the window is the most common sleight of hand in this trade.
See U-values explained for what the figures mean and building regulations for what has to be met.
Where the letter is genuinely useful
Two cases, and they are both narrow.
- Comparing two windows of the same type from different suppliers, where the ratings were produced the same way. Here the letter is a fair shorthand.
- South-facing rooms, where solar gain is a real part of how the room performs and the rating is counting something you will actually experience.
Where it is not useful: as a reason to pay more at the top of the scale. Almost all of the benefit in replacing old glazing happens in the first step, from single or early double glazing to any compliant modern window. The last bands cost disproportionately more for a small gain, and a salesperson leading with the letter rather than the U-value is usually selling the top of the range.
Questions
- Should I insist on the highest band?
- No. Insist on a stated whole-window U-value that meets the current requirement, with the specification written out. The band will follow from that, and you will not be paying for a letter.
- The quote gives a rating but no U-value. Is that a problem?
- It is worth one email. The installer has to certify a U-value in any case, so they have the figure. A quote that will not put it in writing is a quote that has not been specified properly.
- Do doors have energy ratings too?
- They do, under the same kind of scheme, and the same point applies: compliance for a replacement door is a U-value, and the rating is a summary. On a mostly solid door, solar gain barely features, so the two figures tend to agree more closely than they do on windows.
Note This guide is general information about England and Wales, not advice about your specific property. TrustedGlazing is an introducer and does not install, survey or quote. Rely on your installer's survey and your local authority for decisions about your own home.